The OLED diode structure was discovered in 1987, and has been developed into a technology that is used for displays in cellphones, microdisplays and televisions. Microdisplays are small displays that are used under magnification or for projection applications. Some of the applications of microdisplays are camera viewfinders and wearable displays, specifically for professions that require hands free information or for virtual reality applications. Google Glass, currently in development, could usher in mass market microdisplays.
One feature that differentiates microdisplays from normal displays is that the pixel density of microdisplays is much greater than normal displays. An example of normal displays with high pixel densities are the displays on the Samsung Galaxy S4 and the HTC One with 441 and 468 pixels per inch (PPI), respectively. Pixel densities on some microdisplays such as eMagin SXGA and Holoeye HED 6001 have 2646 and 3175 PPI, respectively, about an order of magnitude higher in PPI than normal displays.
Most microdisplays are fabricated on single crystal silicon substrates using CMOS technology. For most microdisplays, liquid crystals (LC) with backlighting are used, but a growing segment of microdisplays are based on organic light emitting diodes (OLEDs). OLED microdisplays have a few advantages over LC microdisplays: OLED is intrinsically simpler in design, does not need backlighting, has a higher viewing angle, faster response time, higher contrast, and it has a higher temperature operating range.
A full color (i.e. RGB colors) OLED display requires patterning to place individual color OLED stacks onto its respective color sub-pixels. Conventional patterning using lithography is not possible with organics due to the sensitivity of the organics to photoresist and chemistries, which is required for photoresist processing. There are few methods to pattern organics other than photoresist such as metal shadow mask, organic vapor jet deposition, and thermal dye transfer. All commercial high resolution full color OLED displays are made using shadow mask patterning in which fine metal masks (FMM) are used.
Current methods produce color emitter layers with a metal shadow mask, which suffers from resolution issues and may prevent color emitter sub-pixel sizes under 10 μm. Other methods create high-resolution OLED microdisplay RGB sub-pixels using a white OLED overlaid with a color filter. The color filters may absorb approximately 80% of the white light and reduce light output from the OLED.